1. Field of the Invention
The present invention relates to a device for introducing a plastic material into a mould, after the mould cavity has been closed, by means of a press plunger which is fitted on a carriage, which carriage is fitted displaceable relative to the mould in order to allow the press plunger, using a first drive, to carry out a first movement with a large stroke at controlled speed and at relatively low pressure, the press plunger being designed to be displaceable with respect to the carriage in order to allow the press plunger, using a second drive, to carry out a second movement with a smaller stroke at a relatively high pressure.
2. Description of the Related Art
Such a device is known from U.S. Pat. No. 4,511,317 which discloses a press plunger which makes use of hydraulic jacks.
The state of the art comprises various kinds of hydraulic and mechanical presses which can cover a long transport path as well as provide a relatively high pressure at the end of the stroke.
However, a number of disadvantages are associated with the use of such systems. Hydraulic systems are always liable to slight leaks, as a result of which it is hardly possible, or only possible with great effort, to fulfil the requirements of a so-called clean room. In the case of mechanical systems, it has hitherto been common practice to use springs. The final pressure is applied to the plastic in a static state since the plastic fluid which has been introduced into the mould cavities is barely compressible. By fitting springs and compressing these to some degree, it is possible in the case of mechanical systems to determine the final pressure. In addition, there are variations in the vertical position of the plungers as a result of tolerances in the amount of plastic introduced.
The disadvantage associated with the use of springs is that, because of friction, compression of the springs always takes place joltingly. As a result thereof, the flow of plastic during the stage of filling the mould is also liable to jolts. Therefore there is a risk that the connection between the chip and the substrate (the gold wires) is damaged. When the mould cavities are being filled, the primary concern is the speed control of the flow, the pressure being of less importance. Moreover, because of the various positions of the springs, the process conditions under which the products are manufactured are never identical and therefore reproducibility is very difficult to achieve. As a consequence of the use of springs in mechanical systems, when moulding is carried the static state, i.e. when the pressure is increased at the end of the stroke, the occurrence of pressure waves in the plastic cannot be prevented, which, on the one hand, is disadvantageous for the end product and, on the other hand, subjects the system to unnecessary forces. In addition, reproducibility cannot be guaranteed. Moreover, it is important with all systems to control the final pressure in order to ensure optimum hardening of the plastic without overloading the device.
The object of the present invention is to avoid the disadvantages mentioned above.